It is desirable to introduce radar absorbing material (RAM) into composite structures such as wind turbine components, for example wind turbine blades. One reason for this is that rotating blades have a radar signature similar to that of aircraft, which can make it difficult for air traffic control and other radar operators to distinguish between aircraft and wind turbines. Incorporating RAM into such structures ensures that the resulting structure has a reduced radar signature that can be distinguished easily from aircraft, and which creates less unwanted events (also known as “clutter”) on the screen of the radar operator.
Existing wind turbine blades are generally manufactured from reinforced composite materials. A typical blade is fabricated in two shells, which are subsequently united to form a single hollow unit. The shells include at particular locations sandwich panel regions having a core of lightweight material such as foam or balsa wood.
By way of background, FIG. 1 shows a cross section of a wind turbine blade 10. The blade 10 is constructed from two aerodynamic shells, upper shell 11 and lower shell 12 which are formed from a glass fibre cloth and resin composite. The shells 11 and 12 are supported by a tubular structural spar 13 formed from glass fibre and carbon fibre.
The spar 13 forms the primary strengthening structure of the blade 10. At the rear of each shell 11 and 12 towards the trailing edge of the blade 10, the shells are formed with a sandwich panel construction, in which a foam core 14 is positioned between sheets or “skins” of glass fibre 15 and 16. The foam core 14 is used to separate the glass fibre skins 15 and 16 to keep the shell stiff in this region.
FIG. 2 shows an exploded sectional perspective view of part of a sandwich panel region of the blade 10. The sandwich panel comprises the foam core 14, which has an inner surface 17 and an outer surface 18. The core 14 is disposed between the inner skin 16 and the outer skin 15. The outer surface 18 of the core 14 and the outer skin 15 face towards an exterior surface 19 (FIG. 1) of the blade 10, whilst the inner surface 17 of the core 14 and the inner skin 16 face towards an interior region 20 (FIG. 1) of the blade 10.
Referring still to FIG. 2, an impedance layer 21 is provided on the outer skin 15, and a conductive ground plane 22, which functions as a radar reflecting layer, is provided between the core 14 and the inner skin 16. The foam core 14 serves as a dielectric layer between the ground plane 22 and the impedance layer 21.
In this example, the impedance layer 21 is a “circuit analogue” (CA) layer, which comprises a carbon-ink circuit printed on an inner surface 23 of the outer skin 15. The carbon-ink circuit is represented by the array of dashes in FIG. 2. For the avoidance of doubt, the outer skin 15 has been made transparent in FIG. 2 so that the CA layer 21 can be seen; in reality, the CA layer 21 would not be visible through the outer skin 15. The CA layer 21 forms a radar absorbing circuit in combination with the ground plane 22. When radar waves are incident upon the blade 10, the combination of the CA layer 21 and the ground plane 22 act to absorb the radar waves so that they are not reflected back to the radar source. In other examples, an otherwise resistive layer may be used in place of the CA layer 21.
Different regions of a wind turbine blade are subject to different forces. Consequently, sandwich panels at different locations within the blade structure may require different core thicknesses. Typically, the core thickness ranges from 5 mm to 45 mm.
The separation between the impedance layer 21 and the ground plane 22 is a key parameter for radar absorption performance, and must be carefully controlled to achieve a blade 10 having the desired absorption properties. Such careful control of the separation of these layers is made more difficult by the varying geometry of the blade 10, specifically the abovementioned variation in core thickness. Theoretical calculations and experimental trials have shown that sandwich panels having a core thickness between approximately 35 mm to 45 mm cannot be turned into high performance RAM using CA or resistive layers and a ground plane arranged as shown in FIG. 2.
A split core arrangement that provides consistent radar absorption performance in structures where core thickness varies is described in WO2010/122351 and WO2010/122352. The split core divides the thickness of the core between inner and outer core layers disposed about an intermediate ground plane. An example of such a split core, and its incorporation within a wind turbine blade, will now be described briefly by way of background to the present invention, with reference to FIGS. 3A to 3C.
FIG. 3A is a plan view of a wind turbine blade 30 of sandwich panel construction and incorporating a split core; FIG. 3B is an enlarged sectional view of a region close to the root 32 of the blade 30, at which point the sandwich panel has a relatively thick core 34; and FIG. 3C is an enlarged sectional view of a region close to the tip 36 of the blade 30, at which point the sandwich panel has a relatively thin core 38.
Referring to FIGS. 3B and 3C, the split core 34, 38 comprises inner and outer core layers 40 and 42 respectively. A ground plane 44 in the form of a layer of carbon veil is located between the inner and outer core layers 40, 42, and the three layers 40, 42, 44 are bonded together by a suitable adhesive. The split core 34, 38 is disposed inboard of a CA impedance layer 46, which is provided on an outer skin 48 of the blade 30.
The thickness of the outer core layer 42, which defines the separation between the impedance layer 46 and the ground plane 44 is the same in both FIGS. 3B and 3C, whilst the thickness of the inner core layer 40 is different. The inner core layer 40 is thicker in FIG. 3B, i.e. closer to the hub 50, than in FIG. 3C, i.e. closer to the tip 36. Since the thickness of the outer core layer 42 remains uniform across the blade 30, a single design of CA layer 46 may conveniently be utilised across the blade 30 providing that the composition of the outer skin 48 is substantially constant across the blade 30. The thickness of the inner core layer 40 does not affect RAM performance, and so this may be chosen to provide the required overall core thickness of the sandwich panel in accordance with the structural requirements of the blade 30 at the specific location of the sandwich panel within the composite structure.
Sandwich panel cores may include a chamfer along one or more edges to avoid stress concentrations from occurring in a laminate structure. The radar absorption performance of single-core arrangements, such as that shown in FIG. 2, tends to be impaired at core chamfers, whereas split-core arrangements, such as those shown in FIGS. 3B and 3C, perform considerably better for reasons that will now be described with reference to FIGS. 4A and 4B.
FIG. 4A shows a chamfered single-layer core 14 of the type shown in FIG. 2, having a thickness of 30 mm and being disposed between an impedance layer 21 and a ground plane 22. FIG. 4B shows a chamfered split core 34, 38 of the type shown in FIGS. 3B and 3C, having an inner core layer 40 that is 20 mm thick and an outer core layer 42 that is 10 mm thick. A ground plane 44 is embedded within the split core 34, 38, between the inner and outer core layers 40, 42, and the split core 34, 38 is located adjacent an impedance layer 46 such that the outer core layer 42 is between the impedance layer 46 and the ground plane 44.
Generally, a reduction in radar absorption performance occurs when the distance between the impedance layer 21, 46 and the ground plane 22, 44 changes from the distance for which the RAM is optimised. In the case of the single-layer core 14 of FIG. 4A, the separation between the impedance layer 21 and the ground plane 22 changes along the entire length of the core chamfer, i.e. between points a and c on FIG. 3A. However, in the case of the split core of FIG. 4B, the separation between the impedance layer 46 and the ground plane 44 remains constant along the majority of the length of the chamfer, i.e. between points b and c in FIG. 4B. The ground plane 44 terminates at point b, so performance is reduced only at the extreme end of the chamfer, i.e. between points a and b in FIG. 4B, rather than along the entire length of the chamfer, i.e. between points a and c, as is the case for the core 14 in FIG. 4A.
Referring again to FIGS. 3B and 3C, it should be noted that the split core 34, 38 includes several parallel slits: a first plurality of slits 52 is provided in the inner core layer 40 and a second plurality of slits 54 is provided in the outer core layer 42. These slits 52, 54 increase the flexibility of the core 34, 38 and enable the core 34, 38 to drape to conform to the required curvature of the blade shell. To avoid disrupting RAM performance, the slits 52, 54 do not penetrate the ground plane 44. To this end, each slit 52, 54 stops short of the ground plane 44.
Whilst the split cores 34, 38 described above perform well in most cases, in certain situations, for example where high drape is required, these cores have been found to be too rigid. This is due to the rigidity imparted to the core 34, 38 by the embedded ground plane 44 and the adhesive layers that bond the ground plane 44 to the respective core layers 40, 42.
Against this background, it is an object of the present invention to provide a more flexible core capable of consistent RAM performance across a wide range of core thicknesses, including relatively thick cores.